Focusing arrangement for cathoderay tubes



R. B. GETHMANN FOCUSI'NG ARRANGEMENT FOR CATHODE-RAY TUBES Sept. 27, 1949.

Filed Feb. 13, 1948 m y PMMMW 0t f .t

ym e t v m Tmwm m Patented Sept. 27, 1949 JEO US NG ABRAN-G EN F RAY TUBES Richard B. Ge

signonto General Ele :tion of New :York

R CA HO E- Bridgeport, Cnn., as-

ric Company, a corporalApplicati n 'Eeb uary 13, .1948, Se i l .No- 8,053 3 Claims. (01. 250161) This invention relates to cathode-ray tubes and'the like and more particularly to improved magnetic arrangements-for 'focussin-g the electron beams in such tubes.

It an object of this-invention to provide a magnetic ioc-ussing-arrangement for cathode-ray ,tubes andthe like whereby the electron beam suchtubes is maintained substantialiyi-n focus, regardless ct -variations in the tube energizing potentials dueto line potential changes.

Yet another object oi this invention is to pro vide-a mag-neticfocussing arrangement for cathode-ray tubes and the like which requires fewer ampere turns, and wherein the loss in focus during the war-in up period is substantially reduced ascompared to -the conventional systems.

'The'teatures-ofnthis invention which are believed'to be new are set-forth with particularity in the appended-claims. The invention itself, however,- together with further objects and -.ad "vantages thereof, may best he understood .by -1'ef-erence= -to the accompanying drawing the singlefig-ure shows a cathodeeray tube .u-tilizinga focussing arrangement in accordance with the present invention.

lt-is contemplated in the presentinvention to provide a hollow cylindrical permanent magnetic member surrounding. the electron zbeamofflacathode-ray tube, member being magnetized in "the direction :of the longitudinal axis zofxthev tube. -It is further contemplated to wind a .0011 about the. circumference of the cylindrical permanent magnet rnember andeto pass a unidirectional cur.- -rent therethrough, hence setting .up an electromagnetic focussing field in the same direction :as the focussing :field idueto the permanent mag.- -netic member.

when an electromagnetic focussi-ng :arrange- .ment alone is utilized, it maybe shown that:

Hence, assuming that the beam accelerating potential and the potential across the electro- -magneticfocussing coil vary linearly with change in line potential, for every changein line "potentiaIthe'beamWiH no longer bein focus, and some magnet provides a magnetic field which focusses the electron beam only for a particular .beam accelerating potential. Hence. when such type of focussing arrangement is used, it is necessary nt on, a combination of the two above-mentioned types o -systems ut ze and .i 'it ee se t y t --E.qn Qn 1. n w becomes:

New t -equa on 9i he ght li e a gen to -the eur-we repr s nt ng Eq io 2 at anoint .11, rand assinsthmll h the O i n 2=\/.151'-;E;E an. V v 511 Th reiore sub titu n n -imi at o K i l 0 2 /E1 Eo 1 *ltihwl i es lilhenthe res stance .of th pf cu coi s made 1 9b tha the cur ntpotential lationship. is;

and when the permanent magnetfield is adjusted so that;

Ihe 1; rrent o vtows he beam a a beama eeleratiner te. ahEis:

ing potential and the potential acrom the focussing coil vary linearly with change in line potential, any increase or decrease in E in Equation is accompanied by a corresponding increase or decrease in the focussing current I of this equation, sufficient to keep the system substantially in focus at all times.

It can be seen, therefore, that assuming E1 to be the normal operating beam accelerating potential, the permanent magnetic field is made such that it focusses the electron beam at an accelerating potential of 2 the electromagnetic coil is made E1 is impressed thereacross, a

The resistance of such that, when current E1 changes to (E1:AE1) due to line potential changes I1 changes to:

and, hence, the beam is substantially still in focus.

Referring now to the figure, there is shown a cathode-ray tube I such as the type used in television receivers and the like. An electron beam is generated at the cathode 2 and directed towards a screen 3. The intensity of the electron beam may be controlled by control electrode 4, and electrodes 5 and 6 may act as accelerating electrodes therefor. The usual beam deflection means for scanning purposes are shown at 1.

In accordance with this invention, intermediate the accelerator electrodes 5 and 6, for example, a cylindrical shaped member 8 is mounted about potential up to the ray tube 1 surrounding the electron beam,

the member 8 being composed of permanent magnet material, and having a coil 9 wound around the periphery thereof. The terminals of coil 9 are connected to a source of unidirectional potential It, the value of which is assumed to vary linearly with the beam accelerating potential due to changes in the alternating current line potential.

The member 8 is magnetized so that a magnetic field is set up in the direction of the longitudinal axis of the tube as previously described. This magnetic field may have a predetermined intensity, in accordance with the previously developed, mathematical analysis. Unidirectional potential is impressed across the coil 9, and the resistance of this coil may similarly be given a predetermined value, as previously described. It is apparent that coil 9 may be wound externally or internally to cylindrical magnet 8, or the two may have separate positions along the axis of tube l.

The present invention therefore provides a simple and inexpensive magnetic focussing device for cathode-ray tubes and the like. Due to the presence of the permanent magnetic field, the ampere turns of the focussing coil may be less than in conventional electromagnetic focussing systems, and in addition the usual drift during the warm-up period in such conventional systems is greatly reduced in the present system due to the above-mentioned permanent magnet field. The improved arrangement for focussing an electron quired for each variation in line potential. In a television receiver employing this improved focussing arrangement, it has been found that satisfactory operation is obtained for changes in line 20 per cent.

While a certain specific embodiment has been shown and described, it will of course be understood that various modifications may be made without departing from the invention. The appended claims are therefore intended to cover any such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a cathode-ray tube having a neck portion, a source of electrons, means for forming said electrons into an electron beam, beam accelerating means, and beam focussing means, said beam accelerating means being connected to a source of unidirectional potential, and said beam focussing means comprising, in combination, a hollow cylindrical permanent magnet of uniform cross section having an inner diameter substantially equal to the outer diameter Of said neck portion and adapted to be supported directly on said neck portion, an electromagnet comprising a coil wound around the outer surface of said permanent magnet, said magnets together providing the total focussing magnetomotive force for said beam, said electromagnet being connected to said previously mentioned source, and the individual magnetomotive forces of said magnets being so correlated that said beam is in focus for a certain unidirectional potential value Of said source, and remains substantially in focus for variations in said unidirectional potential value.

2. In a cathode-ray tube having a neck portion,

.a cathode, an accelerating electrode, and a cathode-ray beam focussing arrangement, said accelerating electrode being connected to a source of unidirectional potential, said focussing arrangement comprising, in combination, a hollow cylindrlcal permanent magnet of uniform cross section having an inner diameter substantially equal to the outer diameter of said neck portion and adapted to be supported directly on said neck portion, an electromagnet comprising a coil wound around the outer surface of said permanent magnet, said magnets together providing the total focussing magnetomotive force for said beam, said electromagnet being connected to said previously mentioned source, the internal resistance of said electromagnet having a predetermined value and the individual magnetomotive forces of said magnets being so correlated that said beam is in focus for a certain potential value of said source and remains substantially in focus for variations in said potential value.

3. In a cathode-ray tube and the like, a cathode, an accelerating electrode, and a cathode-ray beam focussing arrangement, said accelerating electrode being connected to a source of unidirectional potential and having a unidirectional potential E1 impressed thereon, said focussing arrangement comprising, in combination, a permanent magnet and an electromagnet, said magnets together providing the total focussing magnetomotive force for said beam, said electromagnet being connected to said previously mentioned source, and the internal resistance of said electro- V magnet being such that a current K I ="E I l flows therethrough, where K is a constant dependent upon the design of the electromagnet,

and said permanent magnet having such a mag- 10 netomotive force that it focusses said beam at an accelerating potential of being such that said beam is in focus at a potential source value of E1, and remains substantially in focus for variations in said potential value.

RICHARD B. GETHMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 1 2,442,975 on said accelerating electrode, the arrangement Name Date Mynall Oct. 22, 1940 Bradley Dec. 30, 1947 Grundman June 8 1948 

